The present invention relates to the passive venting of a case for a rocket motor or ordnance item, such as a warhead, and more particularly to a new and improved apparatus and method for weakening and effecting rupture of the case in the event it is exposed to a severe thermal threat to prevent a catastrophic explosion or detonation of the propellant or explosive within the case as the result of pressure buildup therein.
Rocket motors or ordnance items present an extreme hazard in the event of “cook-off” which may be defined as the detonation or deflagration of an active material such as an explosive or a propellant charge, owing to external heating in an accidental fire or the like. This hazard is typically minimized by providing a case which contains the charge, with an arrangement for opening the case to vent pressure therein before the explosive or propellant attains a temperature where cook-off may occur. Such a case contributing to cook-off may be a wall, as of a rocket motor or penetrating warhead, directly in contact with the active material but may include structure surrounding the motor or warhead.
Prior art cook-off prevention arrangements include rocket motor cases of reinforced plastic which soften and fail on fast, direct cook-off from exposure to flame before a contained propellant attains cook-off temperature. However, this arrangement is ineffective when the case is subjected to slow cook-off from indirect heating. Another arrangement utilizes a case with stress riser grooves which cause the case to open at the grooves when the case is subjected to pressure by an explosive therein initially decomposing from heat. This arrangement is effective with relatively weak cases on both fast, direct cook-off and slow, indirect cook-off. However with a relatively strong case for target penetration, the stress risers cannot weaken the case sufficiently so that venting either does not occur to prevent cook-off or occurs at such a high pressure that nearby structures are damaged and personnel are injured.
Prior art cook-off prevention arrangements also include a case provided with an orifice having a closure opened or released by melting or thermal stress. Other prior art arrangements include a case vented by thermal stress when one side is heated. This stress may be increased by stiffening and thermally insulating portions of the case. These arrangements, like the stress riser arrangement, are deficient with strong cases. Further prior art arrangements include explosive or other case penetrators activated at a temperature approaching cook-off. These latter arrangements are effective but may themselves be a hazard and require initiation devices which are relatively complex and may be adversely affected by long storage.